Electrical insulators commonly known as suspension insulators may be used individually, but usually form part of a string to support an electrical conductor from a supporting structure. Generally such a suspension insulator comprises two metal hardware members secured to opposite surfaces of a suitably contoured porcelain insulator shell, one hardware member being embedded by means of cement in a cavity in the porcelain shell. By this arrangement the metal hardware members are separated and insulated each from the other. The hardware members, typically an upper cap and a lower pin, each are secured to one of the opposite surfaces of the insulator shell usually by a layer of Portland cement or other suitable material. Typically an insulating, non-conductive glaze will cover the exposed porcelain surface.
High voltage direct current power transmission lines are known to experience cracked suspension insulators after some period of service. This cracking may be caused by an ionic current flow through the moisture in the Portland cement. Because this current flow always is in the same direction in a direct current system, the resulting electrochemical reaction causes the pin to corrode and hence to "grow" if the pin is the positive, or anodic, terminal in the insulator. This "growth", in turn, leads to tensile stresses within the ceramic insulator that produces the cracking phenomenon.
Head cracking problems have arisen on alternating current lines using suspension insulators coated with a semiconducting glaze. The cracking in that AC case has been attributed to increased leakage current due to the semiconducting glaze. As revealed in U.S. patent application Ser. No. 404,620 filed Aug. 2, 1982, now U.S. Pat. No. 4,443,659, head cracking of semiconducting glaze insulators can be greatly reduced or eliminated by use of non-ionic current conducting cementing means. AC suspension insulators used with standard insulating glazes have not generally presented head cracking problems.
According to one theory developed to explain this cracking phenomenon in direct current insulators using standard insulating glazes, adverse environmental conditions, of which moisture and contamination are typical, increase the insulator surface leakage current. When the leakage current reaches the Portland cement, it flows through it, not just over the cement surface, inasmuch as moisture from the environment is always present in the cement, thereby increasing the cement conductivity and enhancing the undesirable ionic electrochemical process of attack upon the galvanized pin.
Thus, there is a need for some means to eliminate ionic current flow through the Portland cement in insulators that have the conventional glaze which is characteristic of the high voltage direct current insulator, to prevent pin growth and consequent insulator cracking.